The following U.S. patent documents are believed to represent the current state of the art: U.S. Pat. Nos.: 6,693,600; 6,476,766; 6,140,975; 6,091,369; 5,936,583; 5,548,827; 5,204,687; 4,876,709; 4,821,040; 4,443,803; 4,366,486; 4,328,501; 4,161,737 and 4,101,898. U.S. Published Patent Application 20030048227.